The newly-discovered Kepler-10b is, in terms of size, the most Earth-like exoplanet yet discovered, and the first completely confirmed discovery of a rocky planet around another star. This is very good news for the hunt for Earth's twin.

NASA's Kepler Mission began focusing most of its resources on the hunt for small exoplanets last year, and already we're witnessing the exciting result. NASA has released this video explaining the new finding, in which Kepler deputy leader Natalie Batalha gives the lowdown on the new planet:

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So how did we find this modest-sized planet? The Kepler probe has an incredibly precise photometer which can measure the minute changes in a star's brightness as a planet passes in front of it. This method has proven very useful in spotting larger exoplanets, but Kepler is the first NASA mission with a photometer powerful enough to pick up on the existence of much smaller, Earth-sized planets. Kepler-10b, at just 1.4 times the size of Earth, is by far the smallest exoplanet we've yet discovered.

Unfortunately, Kepler-10b doesn't fit any of the other criteria for a potentially habitable planet. It's 20 times closer to its star than Mercury is to ours, placing it far from the habitable zone that would allow life to survive there. Indeed, Kepler-10b races around its star, completing an orbit every 0.84 days. And, though it's only 1.4 times the size of Earth, Kepler-10b is a dense world, with a mass 4.6 times that of Earth and an average density similar to an iron dumbbell.

Still, Kepler should be capable of detecting rocky planets inside the habitable zone, and the discovery of Kepler-10b suggests that we really will be able to find a planet out there that is just like Earth, both in terms of size and location. Kepler scientist Douglas Hudgins puts this new find into perspective:

"The discovery of Kepler 10-b is a significant milestone in the search for planets similar to our own. Although this planet is not in the habitable zone, the exciting find showcases the kinds of discoveries made possible by the mission and the promise of many more to come."